A typical two-way radio communication system comprises communication units, communication resources, communication sites, and base station for each of the sites to allocate communication resources among the communication units. Each site has a substantially distinct coverage area and sites are geographically located throughout the system. Each site also has a number of communication resources assigned to it, where at least one of the communication resources is used as a control channel, while a number of the remaining communication resources are used as voice channels. Such systems are known to use both frequency division multiplex access (FDMA) and time division multiple access (TDMA) methods to rebroadcast transmissions.
In a system that supports group (sometimes referred to as dispatch) calls, the communication units are typically arranged into communication groups (talkgroups) and may be located anywhere within the system (in any site). When a communication unit of a talk group requests a group call, it transmits, via a control channel of the site in which it is located, an inbound signaling message to the base station in that site. (It is noted that a group call typically allows all members of the same talk group that are located within the system to communicate with each other.) The inbound signaling message generally comprises the requesting communication unit's individual identification number (i.e., subscriber ID), the requesting communication unit's talkgroup (i.e., talkgroup ID) and a request for a group call. Upon receiving the inbound signaling message, the base station allocates a voice channel to the requesting communication unit's talkgroup.
Thus, in FDMA and TDMA dispatch systems, an outbound link is established and monitored by all units involved in the group call. Of course, each of the communication units are able to receive the communication information on the outbound link because they share a common knowledge about the link such as frequency, timeslot, etc., and can configure themselves to accommodate reception of signals on that link.
FIG. 1 shows a graphical representation of a radio communication system 100 that includes a base station 102 and a plurality of mobile stations, or communication units 103-108, designated as MS1-MS6. These mobile stations communicate with the base station on communication links, e.g., inbound link 109 and/or outbound link 111.
In the last decade, in response to an ever-accelerating worldwide demand for mobile and personal portable communications, spread spectrum digital technology, including one type known as code division multiple access (CDMA), has proved to be an alternative for serving large populations of multiple access users over analog or other digital technologies. The outbound (or forward) link, i.e., between the base station and subscribers, is shared between all users of the systems by means of code division multiplexing. The use of code division multiplexing allows more channels to be derived by the overlaying of carriers one over another and greatly enhances performance in terms of derived channels per hertz of bandwidth. Additionally, security measures are typically applied to prevent inadvertent or unauthorized monitoring of a user's signal by another communication unit receiving the same shared modulation.
FIG. 2 shows a CDMA-based communication system 200 that includes a base station 201 and a plurality of mobile stations, or communication units 203-205. Analogous to the FDM/TDM systems, the mobile stations communicate with the base station via links 207-209, as shown. However, rather than a dedicated link between the base station and each MS, the CDMA system uses a common channel (e.g., frequency) for communications, relying on unique pseudo-random number sequences (PNi for communications directed to MSi) to allow the target MS to decode the transmitted information, as illustrated. It should be noted that, in a typical CDMA system, the inbound links are also shared in CDMA fashion with different PN sequences. The application of a long code, Walsh code, and short code (collectively, the PN) to the coded information-bearing bit stream is referred to herein as the process for spreading of the bit stream with a PN sequence.
CDMA is well suited for cellular communications, but has never been employed in a system supporting dispatch group calls. The difficulties in applying CDMA techniques to dispatch group communications include forward and reverse power control, the need to support soft hand-offs, and provision of security features. With regard to the latter problem, it is well known that today's CDMA systems are required to comply with the TIA/EIA/IS-95-A for 800 MHz operation (cellular) and ANSI J-STD-008 for 1.8 GHz operation (PCS), which include, inter alia, a scrambling code referred to as a long code. This long code is used to provide a layer of security for communication between a communication unit and the base station. As is known, this is accomplished by encoding a data stream with a high-order polynomial string (i.e., long code) that is known only by the targeted subscriber.
In today's CDMA systems, private calls are encoded using the long code, which is typically derived from the subscriber ID (i.e., electronic serial number, or ESN) therefore making the long code unique to each communication unit. Since the ESN is unique to each subscriber unit, the scrambling of the outbound data stream with a portion of this code means that only a target which knows the long code of the intended recipient can decode the transmitted message. In the case of group communications, however, this is no longer a useable approach. That is, in a group communication in which more than one member of the group is in the same cell, each of those members receive the same outbound transmission from the base. This transmission from the base can no longer be scrambled using a long code that is unique to a single subscriber unit, for, in that case, none of the other members of the group receiving that transmission would be able to unscramble it.
Thus, in the context of a group call, which involves more than one subscriber (and hence multiple subscriber IDs), it is not presently possible to provide the layer of security that the IS-95 compliant long code is designed to provide, while still providing an outbound link that may be monitored by all units involved in a group call.
Accordingly, there exists a need to provide group communication services to a plurality of communication units in a spread spectrum system, without foregoing the security features provided in private call services. Further, such a service that complied with IS-95 specifications for CDMA communications would be an improvement over the prior art.